The present invention relates to a character and graphics data display apparatus for performing decoding processing of character and graphics data which are received in encoded form carried by a signal such as a television signal, for example contained in the vertical blanking intervals of a television signal, and for displaying the decoded data.
In recent years, various forms of television broadcasting multiplex systems have been envisaged, which utilize broadcast radio waves. Of these, a television character multiplex broadcasting called the teletext system has been put into practical application for domestic use, whereby character and graphics data concerning news, weather reports, stock market conditions etc. can be displayed by a home television receiver. The term "character and graphics data" as used herein signifies data representing characters and/or numerals, or graphic images, or a combination of these. Another system, called videotext has also been put into practice, in which the public telephone lines are used to transmit character and graphics data to be displayed on television receiver screens.
In the following, a prior art videotext character and graphics data display apparatus of the type described above will be outlined, referring to FIG. 1. In FIG. 1, numeral 1 denotes a character and graphics data decoding section for decoding character and graphics data which are encoded in a television broadcast signal (e.g. encoded within the vertical blanking intervals) and processing the decoded data to produce a video signal which can be applied to drive a television display to display a selected part of that character and graphics data. As described in detail hereinafter, the character and graphics data includes display control data which designates for example the display density with which the character and graphic picture data are to be displayed. A portion of this data which can be displayed at one time on the television screen is generally referred to as a data "page", and the received character and graphics data consists of a plurality of such pages which can be selected by the user as described in the following. Numeral 2 denotes a microprocessor for controlling the character and graphics data decoding section 1 and executing decoding operations. Commands and data are transferred between the microprocessor 2 and other circuits over a bus 20. A receiver circuit 3 serves to extract character and graphics data from a television signal, as digital data. A RAM 4 functions to store the received character and graphics data and to provide a working region for operation of the microprocessor 2. Numeral 5 denotes a ROM which stores a program for controlling the operation of the microprocessor 2. Numeral 7 denotes a display memory, for storing character and graphic picture data that have been decoded by the microprocessor 2. Numeral 6 denotes a display processing circuit for synthesizing an RGB color video signal from the character and graphic picture data that has been stored in the display memory 7. A character generator 9 is utilized during decoding of the character and graphics data into character and graphic picture data. Numeral 8 denotes an auxiliary sound decoder for processing auxiliary sound data which may be contained in the character and graphics data. Numeral 10 denotes a key pad, actuatable by the user for designating selection by the character and graphics data decoding section 1 of specific data, e.g. overall selection of desired character and graphics data, and selection of pages of that data to be displayed. Numeral 20 denotes a television signal receiving section, 21 denotes an antenna, 22 a television signal receiving circuit, 23 an audio demodulator/amplifier circuit, and 24 a luminance amplifier/chrominance demodulator circuit. Numeral 25 denotes a video and audio switching circuit which is operable for selecting either the video and sound signals from the sound demodulator/amplifier circuit 23 and luminance amplifier/demodulator circuit 24, or for superimposing the RGB signal from the character and graphics data decoding section 1 upon the video signal from the luminance amplifier/demodulator circuit 24 while selecting the sound signal from the sound demodulator/amplifier circuit 23, or for selecting the auxiliary sound signal and the RGB signal from the character and graphics data decoding section 1 (in the event that there is auxiliary sound information present and there is no sound signal being produced from the sound demodulator/amplifier circuit 23). The selected sound and video signals thus derived by the selector circuit 25. The resulting output sound signal and output video signal from the video and sound signal switching circuit 25 are respectively supplied to a sound amplifier output circuit 26 and a video amplifier output circuit 27, to drive a loudspeaker 28 and a display device 29 respectively.
The operation of this prior art character and graphics data display apparatus is as follows. A television signal that is received from the antenna 21 is demodulated by the television signal receiving circuit 22 of the television signal receiving section 20, to derive a television sound and video signal respectively. The television video signal is supplied to the receiving circuit 3 of the character and graphics data decoding section 1, whereby character and graphics data are extracted from that video signal. The user can select desired data to be displayed, from among a variety of information constituting the latter character and graphics data, by actuating the keypad 10 to designate to the microprocessor 2 the data which are to be selected. The microprocessor 2 responds by selecting the specified character and graphics data and stores that data in the RAM 4, and also executes decoding processing of the selected character and graphics data in accordance with the stored program held in the ROM 5, to obtain character and graphic picture data which is in a suitable format for subsequent transfer to be displayed by the display device 29, and temporarily stores this character and graphic picture data in the display memory 7. If the character and graphics data contains data which specify as corresponding code values certain predetermined patterns such as shapes of characters or numerals etc (as described hereinafter), then the character generator 9 is utilized in this data decoding processing for generating corresponding character pattern data, as part of the character and graphic picture data that are stored in the display memory 7. The display processing circuit 6 operates on this stored character and graphic picture data to generate the aforementioned RGB color video signal that is supplied to the video and sound signal switching circuit 25.
If the stored character and graphics data in the RAM 4 contain auxiliary sound data, then this is converted by the auxiliary sound decoder 8 into a suitable audio signal for transfer through the video and sound signal switching circuit 25 to the loudspeaker 28.
When a normal television picture is to be displayed, the sound and video signals produced by the television signal receiving section 20 are respectively transferred by the video and sound signal switching circuit 25 to drive the loudspeaker 28 and display device 29 respectively. When character and graphics data are to be displayed, the RGB signal from the character and graphics data decoding section 1 are selected by the video and sound signal switching circuit 25 to drive the display 29. In this condition, auxiliary sound signals are transferred to drive the loudspeaker 28, but only if there is no sound signal component in the television signal being received at that time. Thus the user can select either a normal television picture or a page of character and graphics data to appear on the display device 29.
It can be understood from the above that such a prior art character and graphics data display apparatus has the disadvantage that it is only possible to view (at one time) either a normal television picture or a character and graphics data display. While the user is operating the keypad 10 to select desired character and graphics data, it is not possible for the user to view a television picture. Furthermore, such a prior art apparatus also has the disadvantage that it is only possible to display a single page of character and graphics data on the display device 29 at one time.